docs: def packet(tag, blob):
      """Create small GPG packet."""
      assert len(blob) < 2**32
  
      if len(blob) < 2**8:
          length_type = 0
      elif len(blob) < 2**16:
          length_type = 1
      else:
          length_type = 2
  
      fmt = ['>B', '>H', '>L'][length_type]
      leading_byte = 0x80 | (tag << 2) | (length_type)
      return struct.pack('>B', leading_byte) + util.prefix_len(fmt, blob)